The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit interspersed with layers containing an organic polymer called "kerogen", which upon heating decomposes to produce carbonaceous liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid product is called "shale oil". A number of methods have been developed for processing the oil shale which involve either first mining the kerogen bearing shale and processing the shale on the surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact since the spent shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, one of which is U.S. Pat. No. 3,661,423, issued May 9, 1972 to Donald E. Garrett, assigned to the assignee of this application and incorporated herein by reference. This patent describes in situ recovery of liquid and gaseous carbonaceous materials from a subterranean formation containing oil shale by explosively expanding and fragmenting such formation to form a stationary, fragmented, permeable body or mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Hot retorting gases are passed through the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing "retorted oil shale".
One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishment of a combustion zone in the retort and movement of an oxygen supplying gaseous feed mixture as a combustion zone feed into the combustion zone to advance the combustion zone through the retort. In the combustion zone oxygen in the gaseous feed mixture is depleted by reaction with hot carbonaceous materials to produce heat and a combustion gas. By the continued introduction of the oxygen supplying gaseous feed mixture into the combustion zone, the combustion zone is advanced through the retort.
The combustion gas and the portion of the gaseous feed mixture which does not take part in the combustion process pass through the retort on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called retorting, in the oil shale to gaseous and liquid products and a residue of solid carbonaceous material.
The liquid products and gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid carbonaceous products, together with water produced in or added to the retort, are withdrawn from the retort on the advancing side of the retorting zone. An off gas containing combustion gas generated in the combustion zone, gaseous products of the retorting zone, gas from carbonate decomposition, and the portions of gaseous feed mixture which do not take part in the combustion process is also withdrawn from the retort on the advancing side of the retorting zone.
The off gas, which can contain nitrogen, hydrogen, carbon monoxide, carbon dioxide, methane, water vapor, hydrocarbons, such as methane, ethane, ethylene, propane, propylene and higher hydrocarbons, water vapor and sulfur compounds such as hydrogen sulfide, must be disposed of in an ecologically sound manner. This is primarily because its low fuel value, i.e., less than about 70 BTU per standard cubic foot, can make it uneconomical to use as a fuel. Added to this is the difficulty encountered in initiating and maintaining combustion of fuels with such a low BTU content. Since environmental considerations prohibit discharge of such gas directly to the atmosphere, there is an outstanding need for an economical method of purifying the off gas generated from an in situ oil shale retort.
The present invention is addressed to this problem and, as will be described in greater detail hereafter, is able to accomplish such purification while taking advantage of the heretofore wasted sensible heat remaining in an in situ oil shale retort at the conclusion of the retorting operation.